I have never been able to do a flatlock stitch with 4 threads. You would leave a ridge with that 2nd needle so I do not believe it's possible. Using 3 threads, and some tensions loose (needle) and others tight (lower looper) is what it takes and when you take the seam off your serger it looks really awful with loopy edges hanging out in the air along the edge. But having that "look" means what you did is right and pulling the fabrics apart will give the look you want.
The other thing to remember is when you want the flatlock's "ladder" side to be showing, you have to stitch with the fabric's wrong sides out (right sides together). If you want the loops to show you do it in reverse. (I hope I have said this right as I have not done this in a while.)
Tracy in Nashville, TN
Martha Pullen Licensed Educator